1. Field of the Invention
The present invention relates to an optical mask that shapes a spatial pattern of beam intensity, and an MOPA (Master Oscillator Power Amplifier) laser apparatus including that optical mask.
2. Related Background Art
A hard aperture mask is known as an optical mask that shapes a spatial pattern of beam intensity. In the drawings, the area (a) in FIG. 9 shows a whole view of the hard aperture mask, and the area (b) in FIG. 9 is an enlarged view of section A. The hard aperture mask 80 has an aperture 80a having the same figure (substantially rectangular in an example as depicted) as that of the spatial pattern. In this figure, a hatching area represents a beam shielding section. And, the hard aperture mask 80 transmits through only this aperture 80a, and shields unnecessary beam, thereby shaping the spatial distribution of the laser beam to the same shape as the aperture shape. However, since in such a hard aperture mask the diffraction with edge defining the aperture is large, it is hard to obtain a desired spatial distribution of the beam intensity.
In the spatial distribution of the beam intensity shaped by the hard aperture mask 80, as shown in FIG. 10, a peak of the beam intensity appeared due to the diffraction with the edge defining the aperture. Here, a confocal optical system is constructed by two lenses, and a pinhole of 1 mm in diameter is placed at the position of the focal point. Then, a He—Ne laser beam is irradiated to the aperture of the hard aperture mask, the beam passing through the aperture is Fourier-transformed by a first lens, a high spatial frequency components removed by the pinhole arranged on this Fourier transforming surface, and the beam passing through this pinhole is Fourier-transformed again by a second lens, whereby a spatial distribution of the beam on this Fourier transformed surface is determined. Note that the shaped spatial distribution of the beam intensity, as shown hereinafter, is determined in a similar manner.
For example, in the MOPA laser apparatus, when the laser beam having a peak intensity is amplified in such a spatial distribution, the peak of the beam intensity is further enlarged after the amplification. Then, the peak of this beam intensity may cause the damage to optical components.
Thus, as an optical mask capable of reducing the peak of the beam intensity due to the diffraction as mentioned above, a serrated aperture mask is known as shown in Y. Kitagawa et al., “Prepulse-free Petawatt Laser for Fast Ignitor”, Annual Progress Report 2002, Institute of Laser Engineering, Osaka University, pp. 12-21 (2002). The area (a) in FIG. 11 shows a whole view of the serrated mask, and the area (b) in FIG. 11 shows an enlarged view of section A. A serrated aperture mask 90 includes an aperture 90a having the same schematic shape as the spatial distribution of a beam intensity after a desired shaping, and a fringe defining the aperture 90a is formed in a serrated shape.